DESCRIPTION: The long term objective of this research is to understand the physiological and anatomical mechanisms of the cochlear nucleus (CN) which underlie auditory signal processing. The CN is divided into a dorsal and ventral division (DCN and VCN). In several mammals, the granule cell layer (GCL) and subjacent areas called the small cell cap (SCC) and extemal cell-poor rind encapsulate as a marginal shell the central core of the VCN. The marginal shell is a unique area in that auditory nerve inputs to it are nearly exclusively from low spontaneous rate (SR) fibers. The low-SR AN inputs, together with inputs from collaterals of medial olivocochlear neurons, are postulated to make neurons of the marginal shell optimally suited for encoding the intensity of acoustic stimulus. Presently, little is known about the physiological characteristics of the marginal shell or its neuroanatomical projections. The SCC is considerably enlarged in the human CN, suggesting that the SCC may play an important role in the hearing process particularly in humans. The hypotheses are that the VCN marginal shell is different from the VCN central core in single-unit physiological characteristics regarding SR, maximum response, threshold, dynamic range and slope of the response-level function for pure tone and wideband noise, temporal discharge pattern, excitatory-inhibitory area type, and amplitude-modulated tone encoding properties (Hypothesis 1) and in anatomical projections (Hypothesis 2). Specific aim #1 is to determine basic physiological characteristics of single units in the marginal shell of the cat in terms of the above measures and to evaluate Hypothesis 1. Specific aim #2 is to determine neuroanatomical projections from the VCN marginal shell and core of the cat to targets in the hindbrain and midbrain and to evaluate Hypothesis 2. As a part of Aim #2, we will also evaluate whether or not projections to olivocochlear neurons are more numerous from the VCN shell than from the VCN core. The information to be obtained from this research should ultimately contribute to improving prosthetic devices (cochlear and brain-stem implants and hearing aids) for people with sensorineural hearing impairment.